Every VTOL aircraft faces the challenges of sizing of the engine(s) and the balance of forces. See Daniel Raymer, Aircraft Design: A Conceptual Approach (AIAA Education Series), page 754 (5th ed. 2012).
Vertical take-off can be achieved with a high thrust-to-weight ratio. In contrast, during horizontal flight (cruise), lift forces contribute to the aircraft, and the thrust requirements are much smaller. However, if the intent is to design an aircraft that flies horizontally for a period of time, the VTOL requirement would make the engine requirements too limiting, adding a lot of weight that is then carried around in cruise conditions without functionality. Therefore, the sizing of the engine and thrust matching for a cruise-dominated VTOL aircraft becomes a major issue.
Balance is one of the most important drivers for the design of a VTOL aircraft. During the take-off phase, the thrust has to be distributed around the aircraft, and moments are balanced around the center of mass, in order for the aircraft to remain balanced. The aircraft cannot be balanced if the source of the thrust is in only one location. For example, even when a horizontal aircraft such as the Harrier is balanced in air, the aircraft needs to employ several thrust generating elements in locations specifically chosen in order to cancel out the moments at all times (calculated as force (thrust)×moment arm around the center of the aircraft mass). This is difficult to achieve if the majority of the thrust is located, for instance, in the rear portion of the aircraft (as typically found in a VTOL aircraft).